外键级联更新和删除

--直接玩两个表就能完全理解了
--创建学生表
if(object_id('Tstudent') is not null ) drop table Tstudent;
go
create table Tstudent(
stuno varchar(3) primary key,
stuname varchar(4),
stuclass varchar(3)
)
--创建成绩表
if(object_id('Tgrade') is not null) drop table Tgrade;
GO
create table Tgrade(
grade_stuno varchar(3) ,
grade_lessonno varchar(3) ,
grade  varchar(3)
)

--插入学生数据
insert into Tstudent values('001','gh','101')
insert into Tstudent values('002','lxg','102')
insert into Tstudent values('003','hs','103')
--插入成绩数据
insert into Tgrade values('001','yw','98')
insert into Tgrade values('001','yy','99')
insert into Tgrade values('001','sx','94')
insert into Tgrade values('002','yw','93')
insert into Tgrade values('002','yy','95')
insert into Tgrade values('002','sx','96')

--check
select * from Tstudent
/*
001 gh  101
002 lxg 102
003 hs  103
*/
select * from Tgrade
/*
001 yw  98
001 yy  99
001 sx  94
002 yw  93
002 yy  95
002 sx  96
*/

------------成绩表外键级联更新
alter table Tgrade add constraint FK_StudentNo foreign key (grade_stuno) references Tstudent (stuno) ON UPDATE CASCADE

update Tstudent set stuno='008' where stuname='gh'
select * from Tgrade
/*
008 yw  98
008 yy  99
008 sx  94
002 yw  93
002 yy  95
002 sx  96
*/

-------------级联删除
alter table Tgrade DROP constraint FK_StudentNo
alter table Tgrade add constraint FK_StudentNo foreign key (grade_stuno) references Tstudent (stuno) ON DELETE CASCADE

DELETE FROM Tstudent WHERE stuno='002'
select * from Tgrade
/*
008 yw  98
008 yy  99
008 sx  94
*/


-------------SET 值
if(object_id('Tstudent') is not null ) drop table Tstudent;
go
create table Tstudent(
stuno varchar(3) primary key,
stuname varchar(4),
stuclass varchar(3)
)
if(object_id('Tgrade') is not null) drop table Tgrade;
GO
create table Tgrade(
grade_stuno varchar(3) default('001'), /*Notice*/
grade_lessonno varchar(3) ,
grade  varchar(3)
)

insert into Tstudent values('001','gh','101')
insert into Tstudent values('002','lxg','102')
insert into Tstudent values('003','hs','103')
insert into Tgrade values('001','yw','98')
insert into Tgrade values('001','yy','99')
insert into Tgrade values('001','sx','94')
insert into Tgrade values('002','yw','93')
insert into Tgrade values('002','yy','95')
insert into Tgrade values('002','sx','96')

alter table Tgrade add constraint FK_StudentNo foreign key (grade_stuno) references Tstudent (stuno) ON UPDATE SET NULL
update Tstudent set stuno='008' where stuname='gh'
select * from Tgrade
/*
NULL    yw  98
NULL    yy  99
NULL    sx  94
002 yw  93
002 yy  95
002 sx  96*/
alter table Tgrade DROP constraint FK_StudentNo
alter table Tgrade add constraint FK_StudentNo foreign key (grade_stuno) references Tstudent (stuno) ON DELETE  SET NULL
DELETE FROM Tstudent WHERE stuno='002'
select * from Tgrade
/*
NULL    yw  98
NULL    yy  99
NULL    sx  94
NULL    yw  93
NULL    yy  95
NULL    sx  96
*/
alter table Tgrade DROP constraint FK_StudentNo
truncate table Tgrade;
truncate table Tstudent;
insert into Tstudent values('001','gh','101')
insert into Tstudent values('002','lxg','102')
insert into Tstudent values('003','hs','103')
insert into Tgrade values('001','yw','98')
insert into Tgrade values('001','yy','99')
insert into Tgrade values('001','sx','94')
insert into Tgrade values('002','yw','93')
insert into Tgrade values('002','yy','95')
insert into Tgrade values('002','sx','96')

--SET DEFAULT
alter table Tgrade add constraint FK_StudentNo foreign key (grade_stuno) references Tstudent (stuno) ON UPDATE SET DEFAULT
update Tstudent set stuno='008' where stuname='gh'
/*消息 547，级别 16，状态 0，第 139 行
The UPDATE statement conflicted with the FOREIGN KEY constraint "FK_StudentNo". The conflict occurred in database "njtestdb", table "dbo.Tstudent", column 'stuno'.
The statement has been terminated.
*/
update Tstudent set stuno='008' where stuno='002'
select * from Tgrade
/*
001 yw  98
001 yy  99
001 sx  94
001 yw  93
001 yy  95
001 sx  96
*/
select * from Tstudent

--Clear
drop table Tgrade
drop table Tstudent